On pg. 96 question 10 – PWN Test Prep

On pg. 96 question 10 I’m confused with how you came up with the answer 6, I got the answer -15. How did you come up with 6 I don’t understand?

Same starting graph as the last post, new question:

Find the values of $m$ and $n$ by looking to the graph, which contains the points $(6, -4)$ and $(2, 1)$ .

So $m=-4$ and $n=1$ . Now, $3f(m-n)$ becomes $3f(-4-1)=3f(-5)$ .

Go back to the graph—what’s $f(-5)$ ?